A) An in vitro replication system for saeteriophage N4 has been developed to study the mechanism of its DNA replication. The system mimics in vivo DNA replication. The DNA replication proceeds continuously from both ends of the DNA molecule as in vivo. By using this system, we have purified three DNA replication proteins to homogeneity and identified their functions. B) The Guest Worker (P. Carl) has isolated a mutant which is deficient in RNaseH activity. It has been speculated that RNaseH removes RNA primer from "Okazaki Fragment". We have studied the RNaseH mutant genetically and biochemically. Also, we have been cloning the RNaseH gene to generate a much tighter mutant of RNCseH by in vitro mutagenesis.